Electrodeless discharge lamps operate by using an induction coil to couple electromagnetic energy to a gas mixture, typically a metal vapor and an inert gas, which is enclosed in a sealed vessel. An oscillator is used to generate a high frequency signal which is amplified and delivered to the induction coil. Generally speaking, the lamp operates in two stages. In the start-up stage, the induction coil produces an electric field which ionizes some of the gaseous molecules, creating ions which in turn collide with other molecules, thereby producing further ions. This process continues until the steady-state stage is reached wherein a plasma of circulating charged particles is maintained, primarily by the magnetic field emanating from the induction coil. The stream of charged particles excites the metal vapor atoms, producing radiation, primarily in the UV spectrum, which impinges on a layer of phosphors which coats the walls of the vessel. As a result, the phosphors are excited and produce visible light.
This type of lamp is known as an electrodeless fluorescent lamp. Other types of electrodeless discharge lamps produce visible light directly from the gas contained in the sealed vessel.
Electrodeless discharge lamps, and in particular electrodeless fluorescent lamps, are much more efficient and long-lived than incandescent lamps. An electrodeless fluorescent lamp, for example, has a luminous efficacy of 60-80 lumens/watt, whereas tungsten incandescent lamps typically have a luminous efficacy of only 15-17 lumens/watt. Electrodeless discharge lamps accordingly offer the prospect of very significant energy savings. The development of this technology has been limited, however, by several problems, the foremost of these being the generation of radio frequency interference (RFI). The induction coil acts as an antenna. Even if the lamp operates at frequencies which are approved by the FCC (e.g., 6.78 or 13.56 MHz), the lamp typically generates harmonics of the fundamental frequency which are not within approved wavebands. Another problem has been to minimize losses which occur in the amplification of the high-frequency signal before it is delivered to the induction coil. These problems have been particularly troublesome because the apparatus used to solve them must fit within the confines of an electric light bulb and must not unduly raise the costs of manufacturing the light bulb.
Class E amplifiers are known to be highly efficient, and their use in an electrodeless discharge lamp is described in U.S. Pat. No. 4,245,178 to Justice. The theory underlying Class E amplifiers is described in U.S. Pat. No. 3,919,656 to Sokal et al., which is incorporated herein by reference. The Justice patent, however, describes only a single-ended Class E amplifier and offers no solution to the RFI problem. The single-ended Class E amplifier produces a half sine wave which is rich in harmonics. Moreover, Justice relies on a self-oscillating circuit, containing a feedback winding on a toroidal core, to provide the operational frequency of the lamp. This arrangement does not yield a stable frequency.
The principles of this invention offer a cost-effective solution to both the efficiency and RFI problems.